Well, maybe I am over-analyzing but I'm an engineer and I can't help it! Anyway, I made some measurements. The effective radius of the tire will be the distance from the center of the axle to the ground. It's pretty easy to measure the amount of tire deflection between loaded and unloaded and you can calculate the % difference in effective circumference. For the tires on my '65 - I have low aspect ratio tires - 225/40 18s - they deflect 3/8in when loaded. This results in a 3% error. I would think 70 series tires would have larger deflection so the error would be greater - maybe 5%?
Looking on the tire rack site for same tire dimension the revs/mile calculates to be 4% greater than what you would get using the diameter of the tire.